1. Introduction
This invention relates to an electrolytic plating solution, a process for use of said solution, and to articles formed using the process and solution. In particular, the invention relates to uniformly depositing a coating of electrolytic metal on the walls of a cylindrical opening having a ratio of length to diameter greater than ten to one and a length equal to at least 0.100 inches.
2. Description of the Prior Art
Methods for electroplating articles with metal coatings are well known in the art. Such methods involve passing a current between two electrodes in a plating solution where one of the electrodes is the article to be plated. Using an acid copper plating solution for purposes of illustration, a plating solution would comprise (1) dissolved copper (cupric ions), usually copper sulfate, (2) an acid electrolyte such as sulfuric acid in an amount sufficient to impart conductivity to the bath, and (3) proprietary additives to improve efficiency of the plating reaction and the quality of the metal deposit. Such additives include surfactants, brighteners, levelers, suppressants, etc.
Electrolytic copper plating solutions are used for many industrial applications. For example, they are used in the automotive industry as base layers for subsequently applied decorative and corrosion protective coatings. They are also used in the electronics industry, particularly for the fabrication of printed circuit boards. For circuit fabrication, copper is electroplated over selected portions of the surface of a printed circuit board and onto the walls of through-holes passing between the surfaces of the circuit board base material. The walls of a through-hole are metalized to provide conductivity between circuit layers on each surface of the board.
Early efforts to make circuit boards use electrolytic copper plating solutions developed for decorative plating. However as printed circuit boards became more complex and as industry standards became more rigorous, solutions used for decorative plating were found to be inadequate for circuit board fabrication. A serious problem encountered using electrolytic copper plating solutions involved coatings of uneven thickness on the walls of the through hole with the deposits thicker at the top and bottom of the holes and thinner at the center, a condition known in the art as "dog boning". The thin deposit at the center of the through hole may lead to circuit defects and board rejection.
Dog boning is caused by a voltage drop between the top surface of the hole and the center of the hole. This potential drop is a function of current density, a ratio of the length of the hole to its diameter (aspect ratio) and board thickness. As the aspect ratio and the thickness of the board increase, dog boning becomes more severe due to a voltage drop between the surface of the board and the center of the through hole. This voltage drop is caused by a combination of factors including solution resistance; a difference in surface to hole overpotential due to mass transfer--i.e, a difference in the flow of solution through the hole compared to the movement of the solution over the surface of the board; and a charge transfer difference as a consequence of copper concentration in the hole, the copper to hydrogen ratio in the hole and the concentration of additives in the hole.
The circuit board industry continuously seeks greater circuit densification. To increase density, the industry has resorted to multilayer circuits with through holes or interconnections passing through multiple layers. Multilayer circuit fabrication results in an overall increase in the thickness of the board and a concomitant increase in the length of an interconnection passing through the board. This means that increased circuit densification results in increased aspect ratios and hole length and an increase in the severity of the dog boning problem. For high density boards, aspect ratios typically exceed ten to one.
The prior art, exemplified by Mayer and Barbien, "Characteristics of Acid Copper Sulfate Deposits for Printed Wiring Board Applications," Platinq and Surface Finishing, pp. 46 to 49, March 1981; Malak, "Acid Copper Plating of Printed Circuits," Products Finishing, pp. 38 to 44, March 1981; and Amadi, "Plating High Aspect Ratio Multilayer Boards," PC FAB, pp. 85 to 94, October 1987, all incorporated herein by reference, suggest that increasing the acid to metal ion ratio of an electrolytic plating solution improves plating solution throwing power and deposit distribution. The prior art teaches that the ratio may be altered, for example, by (1) increasing acid concentration while holding metal ion concentration constant or (2) by decreasing metal ion concentration while holding acid concentration constant. However, the prior art also teaches that (1) increased acid concentration may result in anode polarization with cessation of the plating reaction, and (2) decreased metal concentration resulting in exacerbation of the dog boning problem.